In order to measure the viscosity of a fluid substance a certain deformation force can be exerted on this substance and the resistance offered by the substance can be measured. Although blood coagulation is not strictly speaking a viscosity, its measurement can be closely paralleled with that of a viscosity measurement in that a sample is subjected to a particular deformation force and the deformation is observed.
Blood coagulation is the result of a particularly complex biochemical process which is caused according to the type by different combinations of a plurality of components called "factors" contained in the blood. At the present twelve factors are said to play a role in this process.
Interactions between these factors lead in the case of normal blood to the formation of a filamentary network called fibrin, the production of which is characteristic of coagulation. The purpose of the coagulometer is to detect the exact instant of the formation of this network.
The coagulation of blood in the laboratory has been initiated heretofore by several different methods each relying on the use of a respective different group of factors as a function of the information desired. The choice of methods allows one to eliminate or to accentuate the action of different factors in the blood. The sample is either whole blood or plasma. In most cases this blood or this plasma is prevented from coagulating by the addition of an anticoagulant to the fresh blood, this anticoagulant being constituted by a solution of trisodium citrate or sodium oxalate. The plasmas of blood treated with one or the other of these solutions is called "citrated plasma" or "oxalated plasma". These solutions have the effect of eliminating Ca.sup.++ ions from the blood, following which the plasma is separated by centrifuging.
Most coagulometers in use read out the time of incipient appearance of the fibrin. There is a wide variety of electromechanical measuring devices for coagulation in use in laboratories. Many of these devices are rather imprecise and disturb the blood during the coagulation process. Most require a relatively large quantity of blood as the sample and only measure a single point in the coagulation process, not allowing one to measure the evolution of this process. As a rule these devices cannot be used for all coagulation measurements.
Although in many cases the numerical measurement of the process is sufficient, for example for monitoring the administration of anticoagulant substances, it is occasionally useful to know more about the entire evolution of the process, for instance to advance research in this field and to allow more precise diagnoses.
There are so-called "thromboelastographs" which record coagulation graphs. As a result of their complexity such devices cannot be used for running analyses.
The very newest devices meant for use in analytic laboratories are photometric devices which measure the absorption by the sample of a light beam. The formation of fibrin, which is characteristic of coagulation, increasingly diffuses the light of the beam. The remaining light, and as a result the absorbance of the sample, is measured by a photoelectric cell placed across from the source of light, this cell and this source being to opposite sides of the sample.
This analysis system has two advantages over those which have existed to date. No object touches the sample during analysis, and the signal picked up is analog and usable for drawing a graph indicating the evolution of the process.
Nonetheless the basic principle on which this apparatus operates is incompatible with certain analytical methods or for certain pathogenic forms of plasma or blood. The analysis of whole blood is effectively ruled out by the fact that such blood is too opaque. This detection method is in addition poorly adapted for measuring the coagulation of exalted plasmas because they are disturbed during the process. For certain diseases, lipemia for example, the plasma is cloudy, thereby falsifying the measurement. Finally the evaluation of the formation of fibrin is very temporary with such a device.
There are fields other than hematology wherein the measurement of viscosity itself sometimes poses problems that are only inadequately solved, in particular measurement of the viscosity of non-Newtonian liquids. As is known this viscosity is mainly a function of the speed of deformation of these liquids. As a result their measurement can only be made by applying different deformation speeds at a constant temperature so that the viscosity is a curve illustrating a function of these speeds of deformation. In any case the known viscosimeters hardly allow one to measure viscosities at deformation speeds less than 1 sec.sup.-1. On the other hand the determination of the viscosity at a very small deformation speed is of interest because it allows one to determine a molecular weight of a polymer.